1. Field of the Invention
The present invention relates to a driving circuit for a rectifying transistor in a rectifying circuit using a transistor as a rectifying element.
2. Description of the Prior Art
With a rectifying circuit using a transistor as a rectifying element, it is known in the art that a higher efficiency is achieved by virtue of the fact that "on" voltage of the transistor is lower than forward drop-down voltage of a diode.
Referring to FIG. 1 of the accompanying drawings, there is illustrated a conventional basic switching power supply circuit which includes a rectifying circuit using MOS FET as rectifying element.
In FIG. 1, a switching transistor Q3 constituted by a P-channel type MOS FET, and smoothing choke coil L1 are connected in series with each other between an input terminal 1 and an output terminal 2; a smoothing capacitor C1 is provided between the output terminal 2 side end of the choke coil L1 and the ground: and a rectifying transistor Q2 constituted by an N-channel type MOS FET is provided between the switching transistor Q3 side end of the choke coil L1 and the ground. This circuit arrangement forms a back type DC--DC converter.
The switching transistor Q3 and rectifying transistor Q2 have their gates connected to the DR1 drive output terminal and DR2 drive output terminal of a dual-drive type PWM circuit 3b, respectively.
The PWM circuit 3b causes the switching transistor Q3 to be driven to achieve switching action, while at the same time causing the rectifying transistor Q2 to be on-off controlled, as a result of which a stabilized DC output is provided to a load R.sub.L via the output terminal 2.
The switching transistor Q3 and rectifying transistor Q2 are arranged to be alternately turned on and off in succession. Actually, however, the arrangement is made such that when the switching transistor Q3 is turned on while the rectifying transistor Q2 is turned off and vice versa, there occurs such an interval that the switching transistor Q3 and rectifying transistor Q2 are both in the "off" state, for the purpose of preventing an excessive short-circuit current from flowing as a result of both the switching transistor Q3 and the rectifying transistor Q2 being in the "on" state.
Disadvantageously, however, a surge current tends to flow in the circuit when both the switching transistor Q3 and the rectifying transistor Q2 are in the "off" state at the same time.
To prevent flow of such a surge current, it has been the usual practice either to provide a diode D1 in parallel with the rectifying transistor Q2 as illustrated in FIG. 1, or to provide a diode element in parallel with both the switching transistor Q3 and the rectifying transistor Q2.
FIG. 2 illustrates various current waveforms which flow in the DC--DC converter circuit of FIG. 1 wherein the diode D1 is provided in parallel with the rectifying transistor Q2.
When both the switching transistor Q3 and the rectifying transistor Q2 are in the "off" state, a voltage is induced in the the choke coil L1 so that a current I.sub.3 is permitted to flow through the diode D1 toward the choke coil L1.
The flow of the current through the diode D1 will result in a higher power loss therein than that occurs in the rectifying transistor which is in the "on" state, due to forward drop-down voltage which is generated in the diode D1.
Ideally, control should be made in such a manner as to prevent the switching transistor Q3 and rectifying transistor Q2 from being in the "on" state at the same time and from being in the "off" state at the same time. Actually, however, a transistor element has a delay time in operation, and such a delay time represents a dispersion which depends on the type and form of the element.
Thus, in view of the versatileness of the circuit, and in an attempt to prevent both the switching transistor Q3 and the rectifying transistor Q3 from beig in the "on" at the same time, it is required that the output of the PWM circuit 3b which drives the transistors, with such an interval that both the switching transistor Q3 and the rectifying transistor Q2 are in the "off" state.
Furthermore, with increasing output of the switching power supply, the current flowing in the circuit is increased so that the diode D1 should have an enhanced forward current capacity.